Silicon photomultipliers (SiPMs) are becoming of prominent interest in applications where detection of very low light levels and fast timing are required. A SiPM is composed of a large number of identical, small avalanche photodiodes (APD) connected in parallel, each one provided with a quenching resistor connected in series. The quenching resistor value should be in the order of 105-106 Ohm for an effective quenching of the microcell current after an avalanche is triggered (e.g., by a photon or by thermal generation).
Different technologies and designs have been proposed to build a resistor with such specs that can also be easily integrated in the single microcell: polysilicon, thin metal film, vertical bulk silicon, buried silicon. Unless it is perfectly transparent to light or beneath the active area, the quenching resistor should be placed at the border region of the APD cell and have small dimensions (few μm2) in order to minimize the Fill Factor (FF) loss.
Currently, the most common technique to build the resistors is by means of lightly doped poly-Si strips connected to the junction at one end and to the metal layer at the other end. Another way to make the resistors is by means of thin metal films, deposited by using Atomic Layer Deposition (ALD) or Sputtering techniques.
Although both of these techniques allow the realization of quenching resistors with the required specifications in terms of dimension and resistance, they introduce a complication in the fabrication process that negatively impacts the production cost, yield, and reliability. As an example, the complete realization of the poly-Si resistor requires several major fabrication steps including: Poly deposition, Poly doping, and at least 3 photolithographic steps required for resistor/spad contact definition, resistor definition and poly doping enrichment of the polySi/c-Si contact region. Moreover, at least two dielectric layers are needed to isolate the poly layer from both the silicon substrate and the metal grid connecting the SPADs matrix.
Another issue of the poly resistor in certain applications is its high temperature dependence.
Concerning the thin metal film, the main issue is the fact that it is not a standard CMOS process.
The SiPM features low-voltage operation, insensitivity to magnetic fields, mechanical robustness and excellent uniformity of response. Due to these traits, the SiPM has rapidly gained acceptance in the fields of medical imaging, hazard and threat detection, biophotonics, high energy physics and LIDAR.